1. Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly, to an image pickup apparatus including a color separation optical system.
2. Description of the Related Art
Hitherto, when subject luminance is too high in an image pickup system, a predetermined amount or more of light may enter an image pickup element even though a stop is stopped down to a minimum diameter. In addition, when the stop is stopped down, performance degradation may occur due to diffraction phenomena and may cause a problem. Therefore, a neutral density (ND) filter is disposed between an image pickup lens and an image pickup element so as to limit an amount of light entering the image pickup element. As the ND filter, there is often used a glass filter manufactured by melting glass (transparent substrate) having an absorbing substance added thereto, or by applying an organic dye or pigment onto a glass plate (transparent substrate). Because the ND filter manufactured in this method has only a single transmittance, a filter disc including multiple ND filters having different densities is disposed so as to realize an image pickup apparatus having multiple transmittances.
However, when using the filter disc, a space for holding multiple elements and a drive mechanism for switching the filter disc are necessary, and hence downsizing has been difficult. In addition, there has been a problem that a filter holding frame appears as a shadow on a screen when switching the ND filter and a problem that the light amount can be adjusted only step by step.
In order to solve the above-mentioned problems, there is known a transmittance variable filter using a physical property element as a light amount adjusting filter. In particular, a transmittance variable filter using a liquid crystal element can continuously change the transmittance by applying a voltage to the liquid crystal element, and hence it is possible to reduce the number of filters or to eliminate the drive mechanism. On the other hand, because the liquid crystal element has a view angle characteristic, an inclination of a light ray entering the element causes a variation of a spectral transmittance of the light ray. Then, as a result, there is a problem that angle shading occurs in which unevenness is generated in luminance or color in the screen. Note that, the view angle characteristic is caused by a variation in magnitude of a birefringence effect of liquid crystal molecules depending on an incident angle of the light, because an angle between the direction of liquid crystal molecules and the optical axis (of an optical system including a lens apparatus and an image pickup apparatus) varies when the voltage is applied so as to change the transmittance.
Japanese Patent Application Laid-Open No. 2005-227364 discloses a dimmer element in which multiple liquid crystal elements are combined and the elements are driven in a dimmer control range having a small view angle characteristic so as to suppress the view angle characteristic. Japanese Patent Application Laid-Open No. 2005-181512 discloses a method of suppressing an unnatural brightness difference in a taken image by disposing a liquid crystal dimmer element in the optical path so that an alignment direction of the liquid crystal dimmer element becomes substantially parallel to a short side of the imaging surface.
Here, it is known that the image pickup apparatus such as a broadcasting TV camera or a video camera is usually provided with a color separation prism for separating incident light from the lens apparatus into three color light rays, which include a red color light ray, a green color light ray, and a blue color light ray. The color separation prism includes a dichroic film that reflects only specific color light so as to separate the color light in a desired wavelength range. It is known that as the incident angle of light entering the dichroic film becomes larger, the characteristic thereof is shifted more to a short wavelength side, because the dichroic film is formed of a dielectric multilayer film. FIG. 10 illustrates a Philips type three-color separation prism that is generally used. Light rays a and b in FIG. 10 are off-axial rays that form images at opposite image height positions, and the three-color separation prism includes color separation prisms BP, RP, and GP for separating blue color light, red color light, and green color light, respectively. The off-axial ray a in FIG. 10 enters a reflection surface of the blue color light separation prism BP at a smaller incident angle than the off-axial ray b, and enters a reflection surface of the red color light separation prism RP at a larger incident angle than the off-axial ray b. As described above, as the incident angle of the light entering the reflection surface having the dichroic film becomes larger, the characteristic thereof is shifted more to the short wavelength side. Therefore, due to an incident angle difference between the off-axial rays a and b to the prisms BP and RP, white shading occurs in which the screen is colored in an up and down direction.
Similarly, as to a light ray forming an image at a specific image height, light rays passing through an upper side and a lower side of the pupil have different incident angles to the dichroic film. FIG. 11A is a schematic diagram illustrating a manner in which an axial ray passes through the color separation prism and forms an image. FIG. 11A illustrates an upper marginal ray c and a lower marginal ray d. As illustrated in FIG. 11A, the marginal rays c and d have different incident angles to reflection surfaces of the prisms BP and RP. Similarly to the white shading, because the marginal rays c and d have different incident angles to the dichroic film, the spectral characteristics thereof vary. FIG. 11C illustrates a state where the center of the screen is in focus. Because the levels of coloring of the marginal rays are averaged in the in-focus state, the colored state does not occur. In contrast, FIGS. 11B and 11D illustrate unfocused states. Because color shifts of the marginal rays c and d are not averaged but appear as an image in the unfocused state, coloring of a blurred image is generated in which the blurred image is separated into magenta and green on the upper and lower sides.
When the ND filter formed of the element having the angle shading is disposed in a broadcasting TV camera or a video camera having the color separation optical system, it is considered that influences of the angle shading of the element and the dichroic film are overlapped. When the angle shading of the two elements is overlapped with each other, the white shading and the coloring of the blurred image become conspicuous. In particular, because it cannot be expected that how the image is blurred in the screen, the coloring of the blurred image cannot be electrically corrected, which becomes a large problem in a practical manner.